Solo Pros: less communication overhead, rapid decision making, unified direction, do things your way, full ownership
Solo Cons: lack of mental support, heavy burden, all falls on you, context-switching between tasks (e.g., building v. talking to users), in tough times you must find your own support network, less manpower
Cofounder Pros: divvy up roles, combine diverse skillsets, mental support, camaraderie, make use of strengths of multiple people, investors like teams, can boost each other up in tough times, bouncing ideas off each other and brainstorming can lead to breakthroughs
Cofounder Cons: potential for disputes and conflict, compromise can worsen results, must agree and unite, easy to end up with a bad cofounder, must continue to nurture relationship
Common saying: "go fast alone, go farther together"
You can also start solo and bring on cofounders once you feel you need it.